Need help finding an expression related to chemical kinetics!

Hello everyone,

As a chemist, I can use some help related to curve fitting.

Brief description of the problem:

For the analysis of the kinetics of a particular chemical reaction I have obtained the curve as can be seen in the attachment. In this curve x represents time and y represents the decrease in concentration of a particular compound in solution.

Now fur a full analysis I need an expression which can describe this behaviour.

Normally, the behaviour in experiments like these can be described by a combination of exponential factors, for example:

In this case however the beginning of the curve behaves lineary and the end of the curve exponentially as if it is a combination of a pure 0th order decay and a 1st order decay.

Where do I begin to try to find an expression for a curve like this?

I am able to fit a curve using a standard polynomial expression "f(x) = A + A1*x + A2*x2 + A3*x3 + ..." but the 'oscillating' nature of the curve causes to many distortions when I eventually have to take

the derivative of it.

To summarize, is there a standard way where linear behaviour and exponential behaviour are combined in an expression which is able to describe the curvature in the attachment?